Elastomeric thermoplastic polyester polyurethane compositions stabilized against hydrolysis

ABSTRACT

Addition of small amounts of acidic silica to improve hydrolytic stability of polyurethanes.

United States Patent [191 Loew [ Feb. 13, 1973 [54] ELASTOMERICTHERMOPLASTIC POLYESTER POLYURETHANE COMPOSITIONS STABILIZED AGAINSTHYDROLYSIS [75] Inventor: Frederic Christian Loew,

Ridgewood, NJ.

[73] Assignee: Inmont Corporation, New York,

22 Filed: Nov. 27, 1970 [21] App1.No.:93,409

[52] U.S. Cl....260/2.5 AY, 260/2.5 BB, 260/2.5 AK, 260/32.6 N, 260/37N, 260/458 N, 260/459 R, 260/77.5 SS, 264/49 [51] Int. Cl. ....C08g51/56, C08g 51/60, C08g 53/08 [58] Field of Search.....260/77.5 SS, 2.5AY, 2.5 AK, 260/2.5 BB, 37 N, 45.9 R, 32.6 N, 45.8 N;

3,622,526 1 H1971 Zorn ..260/2.5 AK 3,607,822 9/1971 Nishino ..260/37 N3,460,969 8/1969 Murphy ..l 17/ 161 KP 2,740,743 4/ 1956 Pace ..260/2.5AK

3,450,669 6/1969 Nolen ..260/2.5 BB

3,378,517 4/1968 Knipp ..260/37 N 3,296,190 1/1967 Reischl 260/45.7 R3,193,525 7/l965 Kallert ..260/2.5 BB 3,193,522 7/1965 Neumann ..260/2.5BB

FOREIGN PATENTS OR APPLICATIONS 6,716,891 6/ 1968 Netherlands ..260/2.5AY 1,031,799 6/ 1966 Great Britain ..260/2.5 AK

OTHER PUBLICATIONS The Colloid Chemistry of Silica and Silicates; R. K.Iler, Cornell U. Press, N.Y. 1965, pp. 172 and 173. The Science ofSurface Coatings; Chatfield, Ernest Bean Ltd., London, 1962, pp.204-205.

Primary ExaminerDonald E. Czaja Assistant Examiner-C. Warren IvyAttorney-Abner Shefi'er and F. W. Wyman [5 7] ABSTRACT Addition of smallamounts of acidic silica to improve hydrolytic stability ofpolyurethanes.

17 Claims, No Drawings ELASTOMERIC THERMOPLASTIC POLYESTER POLYURETHANECOMPOSITIONS STABILIZED AGAINST HYDROLYSIS The present invention relatesto polyurethane materials, especially microporous polyurethane materialshaving an enhanced resistance to hydrolytic degradation.

The problem of hydrolytic stability of polyurethanes and especially ofpolyesterurethane elastomers has long been known in the art. Onediscussion of this problem is found in Stewart US. Pat. No. 3,463,758 ofAug. 26, 1969, which states that the presence of free carboxylic acid inthe polymer leads to autocatalytic hydrolysis and that, to avoid this,one should make the polyurethane from a polyester having an acid numberof 0.1 or less. Such polyesters of extremely low acid number aregenerally considerably more difficult to prepare and more expensive thanthe ordinary commercial polyesters that are more usually employed forpolyurethane manufacture.

According to one aspect of the present invention it is found that thehydrolytic stability of polyurethane and particularly of polyurethanesheet material (in flat or shaped form) can be improved by incorporatinga small amount of finely divided silica therein. The amount used is lessthan percent, for example in the range up to 8 or 9 percent, but inexcess of 0.1 percent for example in excess of 0.5 percent especially 1percent or above, e.g., 2 percent to 6 or 8 percent. The silicapreferably has a pH of less than 6, e.g., in the range of about 2 to 5.

The use of the acidic silica is particularly advantageous in connectionwith microporous thick polyurethane sheets which do not have theirextensibility constrained by fibrous reinforcement and are useful asupper materials for shoes. These materials have thickness in excess of0.6 mm and elongations at break of 100 percent or more, e.g., at least150 percent especially 250 to 450 percent. The hydrolytic stabilityimparted by the acidic silica is of importance in the use of the shoes,in which the uppers are subjected to hydrolytic influences (e.g., water,perspiration, etc.) as well as in the process of preparing the sheetmaterial in which the polymer is subjected to aqueous coagulating andextracting solutions, often at elevated temperatures.

The disclosure of US. application Ser. No. 819337 filed Apr. 25, 1969,now abandoned, is incorporated herein by reference and attention isparticularly directed to its disclosure as to the utility of theproduct, the nature of the polyurethane, and the nature, treatment anduse of the microporous product.

The invention may be put into practice in various ways and certainexamples will be described to illustrate it. ln this application allproportions are by weight unless otherwise indicated.

EXAMPLE 1 Eight hundred and eighty kgs of pure N,N-dimethylformamidewere placed in a 1,500 Kg reactor flushed with dry nitrogen. 0.027 Kgsof paratoluene sulphonic acid and 0.020 Kgs of dibutyltin dilaurate weredissolved in the dimethylformamide. 205.0 Kgs of Desmophen 2001polyester (a hydroxyl-terminated polyester of 2,000 molecular weight,having a hydroxyl number of about 55.5 mg KOH per g made from about 1mol butanediol-l,4, 1.13 mol ethylene glycol and 2 mols adipic acid),and 48 Kgs of butane diol -l ,4 were than added and dissolved in themixture and the temperature of the mixture adjusted to 25C.

171.6 Kgs of 4,4-diphenylmethanediisocyanate were then added bit by bitcare being taken to keep the temperature from rising above 50C. Once theaddition was complete the mixture was heated to 60C and maintained atthat temperature for 1% hours with stirring. The excess unreactedisocyanate content was then determined by titration of an aliquot.Sufficient butane diol (3.0 Kgs) was then added to react essentiallystoichiometrically with the unreacted isocyanate. The mixture was thenmaintained at 60C with stirring and the viscosity measured periodicallyuntil it had risen to a value of 3500 poise (Brookfield 5 or 6 spindle)as corrected to 24C. 4.10 Kgs of butane diol 1,4 were then added ascapping agent to terminate the reaction dissolved in 3.5 Kgs ofN,N-dimethylformamide. The resultant solution had a polyurethane solidscontent of 32.5 percent.

A solution of polyurethane in N,N'-dimethyl formamide made as describedabove and having a polymer solids concentration of 32.5 percent andcontaining one-half percent based on polymer of N,-(trichloromethylthio) phthalimicle (Fungitrol 11), a fungicide, one-halfpercent based on polymer of tetrakis [methylene3-(3',55',diterti-arybutyl-4'- hydroxy phenyl propionate] methane, anantioxidant for the polyurethane, and 3 percent based on polymer ofSTABAXOL a carbodimide stabilizer, against hydrolysis of thepolyurethane was used.

This solution was mixed into micropulverized sodium chloride (averageparticle size in the range 10 to 15 microns as determined bysedimentation techniques) to give a salt to polymer weight ratio of 1.6to 1. Varying amounts of microscopic silica powder were also added tothe mixture at this stage. The mixture was thoroughly mixed and degassedunder vacuum and then knife coated onto a porous temporary support (asintered high density polyethylene sheet having the properties describedin Example 1 of French Patent 1584466, the disclosure of which isincorporated herein by reference.) The layer on the support was thenimmersed in water at 30C for 1 hour to coagulate the polyurethane. Thematerial was then stripped from the support and immersed'in stationarywater at 60C for 3 hours to remove substantially all the remainder ofthe dimethylformamide and substantially all the salt (e.g., to aconcentration at least as low as 1,000 milligrams of salt per m). Thematerial was then dried at C for 1 hour.

The silica used was of very fine particle size. A number of materialswere used as follows:

a. The material sold as Aerosil R.972 is stated to have an averageparticle size of 20 millimicrons and is described as a micronized silicamade hydrophobic by silicones. It, is believed that the silicones areabsorbed or coated onto the surface of the silica particles. Thematerial is described as a hydrophobic highly dispersed silica acidpowder. 1 gram of the material is stated to contain 3 X 10" primaryparticles and thus has a very large surface area believed to be inexcess of 200 square meters per gram and as much as 400 square metersper gram. A 4 percent by weight dispersion in a l to l methanol waterblend exhibits a pH of 3 .6 to 4.0.

The preparation and properties of the Aerosil R972 are given in detailin the article by Brunner and Schutte in "Chem-iker-Zeitung/ChemischeApparatur 89 (1965) 437-440, which states that this material is made byreacting silica (having silanol groups) with dimethyl dichlorosilane andsteam in a fluidized bed reactor (Fr. pat. 1,368,765;DAS 1,163,784). Thearticle also states that the surface area of the resulting hydrophobicsilica particles is determined by the Brunauer, Emmet and Tellernitrogen adsorption method, the specific area is found to be 120 30m/g', it also states that the chlorine content is 0.04 i 0.01 percent,which indicates that at least part of the acidity of the silica may bedue to small amounts of residual l-lCl dispersed on the surfaces of thesilica particles.

b, c, and d. A material sold as Santocel 54 and materials sold asAerosil A 200 and Aerosil A 300 were also used. These materials are allfumed silicas made, for example by the high temperature hydrolysis of asilicon halide such as silicon tetrachloride. See, for instance, thearticle on Silica ;(amorphous) in the Encyclopedia of ChemicalTechnology Volume 18 (second edition, 1969) page 67; the article onAerosil by Wagner and Brunner in Angew. Chem. 72 (1960) pages 744-750;and the article on Pyrogenic Oxides of silicon and aluminum in the bookUltrafine Particles (ed. Kuhn, Lamprey, Scheer) published 1963 by JohnWiley pages 196-205. The pH of an Aerosil in 4 percent aqueousdispersion is typically about 3.8 (see page 747 of the Watner andBrunner article) and it has less than 0.025% l-lCl, measured byargentometric titration.

e. Another suitable acidic silica is the material known as Gasil 644,which is a micronized silica gel which has been impregnated withmagnesium silicofluoride and which has a particle size of about 3 tomicrons and a pH (measured in 4 percent aqueous suspension) of about2.2. It is believed that this material is prepared by the processdescribed in German patent 957,755 of Feb. 7, 1957. The use of thisparticular silica, which yields particularly outstanding results, is thediscovery of David Price.

Each of these materials was used at concentrations of 0, 1, 3, 5, andpercent by weight based on the polymer.

All were observed to enhance the resistance of the microporous productto hydrolytic degradation as tested by an accelerated treatmentinvolving boiling in water in a pressure cooker and then determiningtensile strength as compared with a sample which had not been boiled.However, this effect was observed to be marked at the 3 and 5 percentvalues but the 10 percent values were not significantly better than thematerial with no silica. A lesser but noticeable improvement at the 1percent value was observed with Aerosil R972. It is thus anticipatedthat values in the range about 1 percent say 2 percent upwards and wellbelow 10 percent, will be most useful. A useful increase in initialmodulus was also observed but no significant adverse effects on theother properties of the microporous sheet were observed.

EXAMPLE 2 The Examples of British Patent Specification No. 28076/69 (nowU.S. application Ser..No. 42,793 filed June 2,1970) were repeated usingthe concentrations of silica quoted in Example 1 above. The silica wasadded only to the substrate formulation and the carbon black pigmentmaster batch was omitted from the substate formulations. The top coatformulation was unchanged. The substrate resin concentration, saltratio, and additive content was the same as for Example 1 above.

Similar enhancements of resistance to hydrolytic degradation wereobserved.

The disclosures of British Patent Specification No. 28076/69 and saidcorresponding U.S. application Ser. No. 42,793 are incorporated hereinby reference.

The acidic silicas in the concentrations described above have also beenobserved to markedly enhance the resistance to hydrolytic degradation ofsolid cast films of these polyurethanes made, for instance, as describedin the just-mentioned applications or in the previously mentionedapplication Ser. No. 8 19,337.

EXAMPLE 3 i A. A solution of elastomeric polyurethane (of I.V.

about 1.1) in dimethylformamide is made in a manner similar to thatdescribed in detail in Example 1 above, and mixed with 0, l, 2 and 3percent (based on the weight of polyurethane) of Aerosil R-972. Eachmixture is cast into the form of a film, from which the solvent isevaporated. The I.V. of each film is then measured; the film is exposedto an atmosphere of percent relative humidity at 100C for 10 hours; theI.V. is measured again;and the ratio of final I.V. to original I.V. foreach film is calculated. The resulting ratios are as follows: 0% silica,0.513; 1% silica, 0.683; 2% silica, O.775;3% silica, 0.763.

Example 3A is repeated, except that the additives and results are astabulated below:

Additive I.V. ratio none 0.537 1.5% PCD 0.500 1.5% PCD, 1% Aerosil R 9720.824 1.5% PCD, 1% Santocel 54 0.767

The PCD used in this example is the same as the Stabaxol used inExample 1. It is triisopropylbenzene polycarbodiimide (see Britishpatent 986,200 Example 1 having the general formula It is also withinthe broader scope of the invention to use other carbodiimides in placeof all or part of the PCD. Examples of such carbodiimides which areknown stabilizers against hydrolysis are found in British patent986,200, whose disclosure of carbodiimides is incorporated herein byreference; that patent describes particularly the use ofpolycarbodiimides having a molecular weight of at least 500 and havingmore than three carbodiimide groups, such as polycarbodiimides in whichthe monomeric unit is a carbodiimide group attached to a divalenthydrocarbon group.

A particularly suitable composition contains the silica together withpolycarbodiimide and a -trichloromethylthio imide as disclosed in mycopending application titled Stabilizing," Ser. No. 93,410 filed Nov.21, 1970, whose entire disclosure is incorporated herein by reference.

The invention has found its greatest utility in the stabilization ofvery high molecular weight thermoplastic elastomeric polyurethaneshaving intrinsic viscosities above 0.8 and preferably 1.0 to 1.4, e.g.,1.1 to 1.2. The polyurethane is preferably produced by the reaction of ahydroxyl-terminated prepolymer with a diisocyanate and a diol. It isalso within the scope of the invention to use a diamine, as a chainextender, in place of the diol, in a manner well known to the art.

The hydroxyl-terminated prepolymer preferably has a molecular weightbelow 6,000 and preferably above 500, more preferably between 800 and2,500; a molecular weight of l,800-2,200 is particularly preferred. Itmay be a polyester of a hydroxy-carboxylic acid (e.g., apolycaprolactone) or a polyester of a glycol and a dicarboxylic acid(e.g., ethylene glycol adipate or 1,4-butanediol adipate) or a mixedpolyester of these types of components. Examples of other dicarboxylicacids which may be used instead of, or in addition to, adipic acid, aresuccinic, pimelic, suberic, azelaic or sebacic acids or aromatic acidssuch as phthalic acid or terephthalic acid. Examples of other glycolswhich may be used to make the polyester are 1,6-hexanediol and1,8-octanediol. The most useful polyesters are aliphatic polyesters inwhich the groups are separated by aliphatic chains averaging about threeto six carbon atoms in length. A

prepolymer which provides flexible or soft segments I in thepolyurethane molecule is preferred. The acid number of the polyester isgenerally below about 3 and preferably less than 2, e.g., 0.2 to 1.5.

The hydroxyl-terminated prepolymer may be a polyether. Typicalpolyethers which are used to provide the soft segments for elastomericpolyurethanes are usually of aliphatic character. One type has theformula H (R0), where R is a divalent alkylene radical, such astetramethylene or ethylene or propylene, and n denotes the degree ofpolymerization.

The preferred diisocyanate is diphenyl methane p,p-diisocyanate, butother diisocyanates may be used as such or in admixture therewith.Examples of other diisocyanates are 2,4-to1uene diisocyanate,p,pdiphenyl diisocyanate and tetramethylene diisocyanate. The mostsuitable diisocyanates have molecular weights below 500.

The chain extender is preferably a low molecular weight glycol. Aparticularly preferred chain extender is tetra-methylene glycol. Othersare ethylene glycol, diethylene glycol, hexamethylene glycol oroctamethylene glycol. Both hydroxyl groups of the glycol are preferablyprimary hydroxyls, and the glycol is preferably unbranched (having nobranches such as dependent methyl or ethyl groups).

In the preferred class of polyester polyurethanes made withdiphenylmethane-p,p'-diisocyanate, those having nitrogen contents in therange of 4 to 5 percent, most preferably in the neighborhood of 4%percent, (e.g., 4.4-4.6 percent have been found to be particularlysuitable.

A particularly suitable polyurethane is made from a polyester prepolymerof at least 1,500 molecular weight, the proportions of aromaticdiisocyanate, polyester and chain extender being such that the highmolecular weight polyurethane is insoluble in 10 percent concentrationin tetrahydrofuran at room temperature.

In preparing the polyurethane it is preferred to use a multistagereaction method in which the proportion of the reactants supplied to theearlier stage, i.e., to the reaction of the hydroxyl-terminated linearprepolymer, diisocyanate and diol chain extender, are such that there isa small stoichiometric excess of isocyanate groups (an excess of lessthan 20 mol percent, e.g., 5 to 15 mol percent) and the reaction iscontinued, in the solvent, until the isocyanate content reaches aconstant level, as shown by analysis of a sample of the reactionmixture, (for instance by titration with a 0.01N solution ofn-dibutylamine in dry tetrahydrofuran). At this time there aresubstantially no unreacted hydroxyl groups in the reaction mixture.Then, in the later stage,. an amount of diol chain extender sufficientto provide one alcoholic hydroxyl group for each unreacted isocyanategroup, as determined by that analysis, is added; the ensuing reaction ofthe isocyanate and hydroxyl groups is continued at controlledtemperature and the viscosity of the mixture is measured during thisreaction until a viscosity corresponding to an intrinsic viscosity inthe range of about 0.9 or 0.95 to 1.4 is reached. At this time anend-capping reagent, such as an alcohol (e.g., methanol or butanediol)or other chain-terminating reactant is added to stop the reaction.

The amount of diol chain extender supplied to the later stage is below20 mol percent (e.g., in the range of about 5 to 15 mol percent) of theamount of chain extender present in the earlier stage.

As disclosed in said application Ser. No. 819,337, th preferredsolutions of polyurethane have a viscosity at least 400 poises, such as2,000-3,500 poises. These viscosities are of course measured on thepolymer solution as such (i.e., in the absence of added silica. asdescribed herein).

The stabilized polyurethanes in accordance with this invention areespecially useful for the production of shoe upper material whose baseis a microporous sheet consisting essentially of elastomericpolyurethane material. Unlike conventional leather substitutes whichhave ultimate elongations of some 20-40 percent, these sheets do nothave their extensibility constrained by the presence of a reinforcingfabric (such as a woven or non-woven fibrous fabric) and can bestretched well over 50 percent (e.g., well over 100 percent and usuallywell over 200 percent). In a preferred form of the invention thesolution of the high molecular weight elastomeric polyurethane is mixedwith finely divided pore-forming microscopic particulate material(preferably microscopic sodium chloride particles) the mixture is shapedinto sheet form and treated so as to add water to the shaped mixture soas to coagulate the polyurethane (which although soluble in DMF isinsoluble in a DMF-water mixture containing some 12 percent water). Thecoagulated sheet is then treated to remove al the pore-formingparticles, e.g., by leaching with hot water, to dissolve out all thesalt.

For use as shoe upper material, the cast thickness is preferable suchthat after coagulation, leaching and drying the resulting microporoussheet is about 0.6 to 2 mm thick.

For use in making shoe upper materials the preferred polyurethanes havemelting points of at least lC preferably above 150C (e.g., about l70to200C, as measured by differential thermal analysis or differentialscanning calorimetry). When formed into a smooth void-free thin film0.2-0.4 mm in thickness they have the properties described below: atensile strength of at least 210 kilograms per square centimeter(preferably at least 350, e.g., about 420 to 560), a percent elongationat break of at least 300 percent (preferably at least 400 percent, e.g.,about 500 to 700 percent), an elastic modulus of at least 105 kilogramsper square centimeter (preferably at least 350, e.g., about 560 to 770),a 100 percent secant modulus (stress divided by strain at 100 percentelongation) of at least 28 kilograms per square centimeter (preferablyat least 84, e.g., about 110 to 134). These mechanical properties aremeasured by ASTM D882-67.

While the mechanism by which the acidic silica aids in the stabilizationof the polyurethane is not clearly understood, it is believed that theacidity of the silica serves to lower the pH of the hydrolyticenvironment, as by counteracting the effect of alkaline materials thatmay be present, or may be formed, in the aqueous medium with which thepolyurethane comes in contact during the production of the microporousmaterial (e.g., during the steps of coagulating the layer ofpolyurethane with aqueous medium and leaching the dispersed salttherefrom) or during use (e.g., when perspiration from the foot of theuser penetrates into the polyurethane layer). The acidity of thepreferred silica materials is extractable by leaching; thus, after anextended Soxhlet extraction of the Aerosil R972 material with water thepH of the silica, initially acidic, became alkaline (e.g., pH 7.7).Also, it is believed that mineral acid carried by the silica mayinteract with the carbodiimide stabilizer which is preferably present,as by catalyzing the reaction of the carbodiimide with water, or withany carboxylic acid end groups, thereby increasing the stabilizingactivity of the carbodiimide. The invention, in its broader scope,accordingly encompasses the use, in place of thesilica, of othersubstantially colorless inert water-resistant fillers that are ofmicroscopic particle size and that carry water-extractable mineral acidmaterial preferably in amount such that the pH of the tiller is in therange of about 2 to 5.

It is also within the broader scope of the invention to employ theacidic particles in only part of the microporous sheet material, relyingon their protective effect on the remainder, which is free of the acidicparticles or has a lower concentration thereof. Thus the acidic silicamay be present in one or more relatively thin microporous polyurethanelayers at or near one or both surfaces of the sheet, while the rest ofthe thickness of the sheet is microporous polyurethane containing less,or no, acidic silica.

It is understood that the foregoing detailed description is given merelyby way of illustration and that variations may be made therein withoutdeparting from the spirit of the invention. The Abstract given above ismerely for the convenience of technical searchers and is not to be givenany weight with respect to the scope of the invention.

lclaim:

1. An elastomeric thermoplastic polyester polyurethane containing acarbodiimide stabilizer against hydrolysis of the polyurethane andfurther stabilized against hydrolysis by the presence of microscopicparticles of acidic silica in amount which is sufficient to in creasethe hydrolytic stability beyond that imparted by said carbodiimide andis above 0.1 percent and up to 8 percent, based on the weight of thepolyurethane, said silica having a pH in the range of about 2 to 5.

2. A stabilized polyurethane as in claim 1 containing a polycarbodiimidehaving a molecular weight of at least 500 and having more than threecarbodiimide groups.

3. A stabilized polyurethane as in claim 2 in which the proportion ofcarbodiimide is in the range of about 0.5-5 percent, and the proportionof silica is about 1 to 5 percent, based on the weight of polyurethane.

4. A stabilized polyurethane as in claim 1, said polyurethane being athermoplastic polyester polyurethane having an intrinsic viscosity above0.8.

5. A stabilized polyurethane as in claim 1, in which said polyurethaneis in the form of a microporous layer.

6. A stabilized polyurethane as in claim 5, in which said polyurethaneis in the form of a microporous selfsupporting sheet consistingessentially of said stabilized polyurethane, said sheet being about 0.6to 2 mm thick and being suitable for use as a shoe upper.

7. A stabilized polyurethane as in claim 6, said polyurethane being athermoplastic polyester polyurethane having an intrinsic viscosity above0.8.

8. A stabilized polyurethane as in claim 7, in which said polyurethaneis a thermoplastic reaction product of a linear polyester having, in itspolymer chain, carboxylic ester groups separated by aliphatichydrocarbon chains averaging about three to six carbon atoms in length,with a diisocyanate and a difunctional isocyanate-reactive chainextender, said polyurethane being soluble in dimethyl formamide.

9. In the process for the production of microporous self-supporting shoeupper sheet material about 0.6 to 2 mm thick of thermoplasticelastomeric polyester polyurethane by mixing a solution of saidpolyurethane in dimethylformamide with microscopic water-soluble saltparticles thereby producing a paste, forming a layer of said paste,coagulating said layer by the action of aqueous coagulant and extractingsaid salt particles with water, the improvement which comprisesincorporating about 1 to 6 percent, based on the weight of the polymer,of microscopic particles of acidic silica having a pH of about 2 to 5into said paste to increase the resistance of said sheet material tohydrolysis.

10. A mixture of the composition of claim 1 and dimethylformamide, thepolyurethane of said composition being thermoplastic, having anintrinsic viscosity above 0.8 and being present in solution in saiddimethylformamide and in concentration in the range of about 20 to 40percent of the combined weight of the polymer and the solvent.

11. A composition as in claim 1 in which the acidic silica is a fumedsilica which is hydrophobic due to the presence of a silicone at thesurface of the silica particles.

12. A composition as in claim 11 containing triisopropylbenzenepolycarbodiimide in amount in the range of about 0.5-5 percent and N-(trichloromethylthio )phthalimide.

13. Process as in claim 9 in which said solution of said polyurethanehas a viscosity of at least 400 poises in the absence of said silica.

14. Process as in claim 13 including the step of mixing said silica andsaid salt particles with said solution.

15. A mixture as in claim 10 in which said po1yurethane solution has aviscosity, in the absence of said stabilizers, of over 400 poises.

16. A mixture as in claim 10 in which said viscosity is 2000 to 3500poises.

17. A mixture as in claim 10 which contains microscopic water-solublesalt particles in amount sufficient to form a paste with said solution,said mixture being adapted to formed into a microporous sheet materialby forming said paste into a layer, coagulating said paste with aqueouscoagulant and extracting said salt particles with water.

1. An elastomeric thermoplastic polyester polyurethane containing acarbodiimide stabilizer against hydrolysis of the polyurethane andfurther stabilized against hydrolysis by the presence of microscopicparticles of acidic silica in amount which is sufficient to increase thehydrolytic stability beyond that imparted by said carbodiimide and isabove 0.1 percent and up to 8 percent, based on the weight of thepolyurethane, said silica having a pH in the range of about 2 to
 5. 2. Astabilized polyurethane as in claim 1 containing a polycarbodiimidehaving a molecular weight of at least 500 and having more than threecarbodiimide groups.
 3. A stabilized polyurethane as in claim 2 in whichthe proportion of carbodiimide is in the range of about 0.5- 5 percent,and the proportion of silica is about 1 to 5 percent, based on theweight of polyurethane.
 4. A stabilized polyurethane as in claim 1, saidpolyurethane being a thermoplastic polyester polyurethane having anintrinsic viscosity above 0.8.
 5. A stabilized polyurethane as in claim1, in which said polyurethane is in the form of a microporous layer. 6.A stabilized polyurethane as in claim 5, in which said polyurethane isin the form of a microporous self-supporting sheet consistingessentially of said stabilized polyurethane, said sheet being about 0.6to 2 mm thick and being suitaBle for use as a shoe upper.
 7. Astabilized polyurethane as in claim 6, said polyurethane being athermoplastic polyester polyurethane having an intrinsic viscosity above0.8.
 8. A stabilized polyurethane as in claim 7, in which saidpolyurethane is a thermoplastic reaction product of a linear polyesterhaving, in its polymer chain, carboxylic ester groups separated byaliphatic hydrocarbon chains averaging about three to six carbon atomsin length, with a diisocyanate and a difunctional isocyanate-reactivechain extender, said polyurethane being soluble in dimethyl formamide.9. In the process for the production of microporous self-supporting shoeupper sheet material about 0.6 to 2 mm thick of thermoplasticelastomeric polyester polyurethane by mixing a solution of saidpolyurethane in dimethylformamide with microscopic water-soluble saltparticles thereby producing a paste, forming a layer of said paste,coagulating said layer by the action of aqueous coagulant and extractingsaid salt particles with water, the improvement which comprisesincorporating about 1 to 6 percent, based on the weight of the polymer,of microscopic particles of acidic silica having a pH of about 2 to 5into said paste to increase the resistance of said sheet material tohydrolysis.
 10. A mixture of the composition of claim 1 anddimethylformamide, the polyurethane of said composition beingthermoplastic, having an intrinsic viscosity above 0.8 and being presentin solution in said dimethylformamide and in concentration in the rangeof about 20 to 40 percent of the combined weight of the polymer and thesolvent.
 11. A composition as in claim 1 in which the acidic silica is afumed silica which is hydrophobic due to the presence of a silicone atthe surface of the silica particles.
 12. A composition as in claim 11containing triisopropylbenzene polycarbodiimide in amount in the rangeof about 0.5- 5 percent and N-(trichloromethylthio)phthalimide. 13.Process as in claim 9 in which said solution of said polyurethane has aviscosity of at least 400 poises in the absence of said silica. 14.Process as in claim 13 including the step of mixing said silica and saidsalt particles with said solution.
 15. A mixture as in claim 10 in whichsaid polyurethane solution has a viscosity, in the absence of saidstabilizers, of over 400 poises.
 16. A mixture as in claim 10 in whichsaid viscosity is 2000 to 3500 poises.